1. Field of the Invention
The present invention relates to arrangements for switching data packets in switched local area networks, in particular to arrangements for cascading multiple multiport network switches to increase the number of ports in a network switching arrangement.
2. Background Art
A multiport network switch in a packet switching network is coupled to stations on the network through its multiple ports. Data sent by one station on a network to one or more other stations on the network are sent through a network switch. For example, commonly-assigned U.S. Pat. No. 5,953,335 discloses a network switch configured for switching layer 2 type Ethernet (IEEE 802.3) data packets between different network nodes. The network switch determines the destination of a received data frame from the data frame header. The network switch then transmits the data frame from the appropriate port to which the destination network station is connected.
A single Ethernet network switch may have a number of 10/100 Mbps ports, equaling, for example, 12 ports. The number of end stations connected to the single network switch is limited by the number of ports (i.e., port density) on the network switch. However, today""s users of networking devices demand flexibility and scalability without such constraints. To address this need, manufacturers have developed modular architectures that enable cascading of identical networking devices or network switch modules. By cascading these devices in a loop, port density can be readily increased without redesign or development of costly interfaces.
Unfortunately, an increase in the number of cascaded network switch modules to increase port density does not necessarily result in an increase in the capacity of network addresses that can be stored within the cascaded network switch modules. In particular, each network switch module typically includes an address forwarding table that specifies for a given network address (e.g., a media access control (MAC) address) a network switch port that serves the corresponding network address. Hence, a network switch can determinate a destination output switch port for a received data frame by searching the address forwarding table using the destination MAC address within the received data frame.
In a cascaded arrangement, however, a network switch module that does not have a network switch port that serves a network station having a certain MAC address may have an address table entry for the certain MAC address that specifies an expansion port that connects of the multiple switch modules in the cascaded arrangement. Hence, the address table specifies for the certain MAC address that the data packet should be forwarded to another one of the network switch modules of the cascaded arrangement. The one switch module that serves the network station having the certain MAC address will have within its address forwarding table an address table entry that specifies the certain MAC address and the corresponding destination output port. Hence, at least two address table entries are used to locate a single MAC address.
Consequently, the cascading of multiple network switch modules may increase port density, but results in no increase in the capacity of stored network addresses, since for each added network switch module in the cascaded arrangement, another address table entry is needed for each MAC address specifying whether a received data frame should be output onto the expansion port. Hence, the total capacity of stored network addresses remains limited to the number of address entries that can be stored by any one network switch module. Hence, above-described cascaded arrangement is not scalable as more network switch modules are added.
There is a need for an arrangement that enables multiple network switch modules to be cascaded for increasing port density, without limiting the number of network devices that may be serviced.
There is also a need for an arrangement that enables multiple network switch modules to be cascaded, without increasing the size of address forwarding tables.
There is also a need for arrangement that enables multiple network switch modules to be cascaded in a manner that provides scalability for port density and memory capacity for address forwarding tables configured for storing switching information for stored network addresses.
These and other needs are attained by the present invention, where a network switching system having a plurality of multiport switch modules arranged in a cascaded sequence uses a signaling protocol that eliminates the necessity of storing a given network address within each of the address forwarding tables of the multiport switch modules. A network switch module, having an address forwarding table for storing switching information for respective stored network addresses and that receives a data packet, outputs a switching request to a subsequent one of the switch modules based on a determined absence of the destination address of the received data packet in the address forwarding table. Each of the successive network switch modules passes the switching request to the next switch module in the sequence upon a determined absence of the destination address in the corresponding address forwarding table. The switching request identifies the originating network switch module having received the data packet, enabling the network switch module having the last position in the cascaded sequence relative to the originating network switch module to generate a flood signal to all the network switch modules if none of the network switch modules have the destination address in their respective address forwarding tables. Hence, the multiport switch modules can be arranged in a cascaded sequence to provide increased port density, without the necessity of unnecessarily populating the address forwarding tables with each and every network address detected by any one of the network switch modules.
One aspect of the present invention provides a method in a network switch module connected in a cascaded sequence with a plurality of other network switch modules. The method includes receiving on a first of network switch ports in the network switch module, from a network medium, a data packet that specifies a source address and a destination address, and selectively adding to an address forwarding table of the network switch module the source address and an identifier of the first network switch port based on a determined absence of the source address in the first address forwarding table. The method also includes outputting a switching request, for switching of the data packet, on a bus to a subsequent one of the other switch modules in the cascaded sequence based on a determined absence of the destination address in the address forwarding table. All the network switch ports are selectively flooded with the data packet in response to a flood signal from a preceding one of the other switch modules in the cascaded sequence. The outputting of the switching request enables each of the other network switch modules to determine the presence of the destination address in their respective address forwarding tables, without unnecessarily populating their address forwarding tables with network addresses having unknown origins. Hence, the duplicate entries of network addresses in multiple address forwarding tables can be eliminated. In addition, the selective flooding in response to a flood signal enables the cascaded network switch modules to attempt to access an unknown destination address without unnecessarily populating the address forwarding tables.
Another aspect of the present invention provides a method in a network switch module connected in a cascaded sequence with a plurality of other network switch modules. The method includes receiving, from a preceding one of the other network switch modules in the cascaded sequence, a switching request having a data frame identifier that specifies at least a destination address for a received data frame and one of the other switch modules having received the received data frame. The method also includes selectively outputting, to a subsequent one of the other switch modules in the cascaded sequence in response to a determined absence of the destination address within an address forwarding table of the network switch module, one of the switching request and a flood signal based on a position of the network switch module in the cascaded sequence relative to the one switch module having received the data frame, the flood signal requesting flooding of all network switch ports of the other network switch ports with the received data frame.
Still another aspect of the present invention provides a network switch module configured for connection with other network switch modules in a cascaded sequence. The network switch module includes a plurality of network switch ports, an address forwarding table configured for storing an address entry having a network address and a network switch port identifier for a network station coupled to one of the network switch ports, and switching logic. The switching logic is configured for determining the presence of the address entry in the address forwarding table for a destination address of a received data frame. In response to a determined absence of the address entry for the received data frame, the switching logic selectively outputs to another one of the switch modules a switching request for the received data frame or a flood signal for outputting the received data frame on all network switch ports, based on a position of the multiport switch module, determined by the switching logic, relative to the one of the multiport switch modules having received the data frame. The network switch module also includes a data bus interface configured for connecting the network switch module with the other network switch modules in the cascaded sequence and transferring the switching request and the flood signal.
Additional advantages and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The advantages of the present invention may be realized and attained by means of instrumentalities and combinations particularly pointed in the appended claims.